Many rocks studied previously were probably deposited in acidic water.

While this would not have precluded the possibility of micro-organisms taking hold on Mars, it would have been more challenging, scientists believe.

Identifying clays shows there were at least some locations on the planet billions of years ago where environments would have been much more favourable.

"We have found a habitable environment that is so benign and supportive of life that probably if this water was around and you had been there, you would have been able to drink it," said John Grotzinger, Curiosity's project scientist.

The rover drilled a powdered sample from a mudstone at its exploration site in Gale Crater, a deep impact bowl on Mars' equator.

This was delivered to the two big onboard laboratories, Sam and Chemin, for analysis.

The rock sample was found to contain 20-30% smectite - a particular group of clay minerals.

Their high abundance and the relative lack of salt are strongly suggestive of a fresh-water environment for the mudstone's formation.

The presence of calcium sulphates, rather than the magnesium or iron sulphates seen in previous rock analyses at other locations on the planet, adds to the evidence that the sampled rock in Gale was deposited in a neutral to mildly alkaline pH environment.

Scientists think Curiosity probably drilled into an ancient lakebed.

In addition to sulphur, nitrogen, hydrogen, oxygen, phosphorus and carbon - some of the key chemical elements for life - it found compounds in a range of oxidised states, meaning there were electrons moving through the environment. Those could have been co-opted as an energy source by simple life-forms, if they ever existed in Gale.

"What we've learned in the last 20 years of modern microbiology is that very primitive organisms - they can derive energy just by feeding on rocks," explained Prof Grotzinger.

"Just like on battery - you hook up the wires and it goes to a lightbulb and the lightbulb turns on. That's kind of what a micro-organism would have done in this environment, if life had ever evolved on Mars and it was present here."

The rover is assembling quite a catalogue of water evidence in the crater.

Already, it has seen the remains of an ancient riverbed system, where water once flowed perhaps a metre deep and quite vigorously.

The picture that seems to be emerging is one where sediments were transported downhill from the eroding crater rim into a network of streams that then flowed into the lake environment represented by the mudstone.